Conventionally, image forming apparatuses employing an electrophotographic system such as a copier, a printer, and a facsimile machine have been widely known. In such image forming apparatuses, an electrostatic latent image is formed on a surface of a photoreceptor. The electrostatic latent image is developed with toner, and a toner image thus obtained is transferred and fixed onto a sheet such as a paper sheet or the like. The toner used to develop the electrostatic latent image is supplied from the developing device to the surface of the photoreceptor. To the developing device, toner is supplied from a toner cartridge via a toner transport device. In a structure in which a toner cartridge is disposed over a developing device, a toner transport device is provided with a toner transport path extending in an up-and-down direction.
On the other hand, in recent years, with the improvement in high image quality of image forming apparatuses, particles of toner have been designed to have a micro diameter. A toner having such micro particles generally does not have good fluidity, so that retention and solidification of toner easily occur in a toner transport path of a toner transport device. In order to overcome such a problem, a toner transport device needs some contrivances.
For example, an apparatus disclosed in patent document 1 (Japanese Unexamined Patent Publication, No. 174467/1992 (Tokukaihei 4-174467, publication date: Jun. 22, 1992) is arranged such that four developing devices for different colors are disposed in the up-and-down direction and toner is supplied from a toner hopper (equivalent to a toner cartridge) to the developing devices. The developing devices are movable in upward and downward directions so as to face a photoreceptor when used. To enable such movements, an accordion pipe is used for a toner transport path through which toner is supplied from the toner hopper to the developing devices. Further, a rotator is provided by the accordion pipe. The rotator has a plurality of protrusions provided thereon in its circumferential direction, and is driven by a motor so as to rotate. That is, in the patent document 1, by rotary motion of the rotator, the protrusions disposed on the rotator repeatedly hit the outer surface of the accordion pipe, causing toner remaining in pleats of the inner surface of the accordion pipe to fall off.
Further, patent document 2 (Japanese Unexamined Patent Publication, No. 296731/2001 (Tokukai 2001-296731, publication date: Oct. 26, 2001) discloses an apparatus in which four developing devices for different colors are arranged in a lateral line and supplied with toner from respective toner cartridges via a first transport path and a second transport path. Inside the second toner transport path disposed in an up-and-down direction (vertical direction), a spring agitator is provided that moves in upward and downward directions. The movement of the spring agitator corresponds to rotation of an auger, which is provided inside the first transport path disposed in the horizontal direction. That is, in the patent document 2, toner is prevented from adhering to the inner wall of the second transport path, by causing the spring agitator to move in the upward and downward directions inside the second transport path.
However, the conventional structures have difficulties in properly preventing retention and solidification of toner in a toner transport path disposed in the up-and-down direction.
Specifically, in the patent document 1, since the toner transport path disposed in the up-and-down direction is an accordion pipe, structurally, the transport path does not easily prevent retention and solidification of toner inside the toner transport path. In addition, since the rotator gives impacts and vibration to part of the toner transport path, toner easily remains and solidifies in a portion away from the rotator.
Further, in the patent document 2, since the spring agitator is disposed inside the second transport path disposed in the up-and-down direction, toner easily remains and solidifies on the surface of the spring agitator. That is, the spring agitator itself tends to become a cause of restricting toner transport.